example.txt

Sarah, Alice, John, Bob

Sarah -400
---
Sarah -300
Alice 100
John 100
Bob 100

John -100
---
Sarah -300
Alice 150
John 0
Bob 150

ledger.rb

class Ledger
  def initialize names, values=[]
    @ledger = {}
    names.each.with_index do |name, i|
      @ledger[name] = 0
      @ledger[name] = values[i] unless values.empty?
    end
  end
  
  def transaction amount, buyer, beneficiaries
    @ledger[buyer] -= amount
    share = amount / beneficiaries.size
    beneficiaries.each { |person| @ledger[person] += share }
  end
  
  def report
    payments = []
    
    until @ledger.values.all? { |v| v == 0 } do
      @ledger.each do |name, balance|
        next if balance == 0
        if balance < 0
          payer_name, payer_balance = find_payer
          amount = [balance*-1, payer_balance].min
          @ledger[name] += amount
          @ledger[payer_name] -= amount
          payments << "#{payer_name} pays #{name} #{amount}"
        end
      end
    end
    
    payments
  end
  
  private
  
  def find_payer
    @ledger.find { |name, balance| balance > 0 }
  end
end

ledger = Ledger.new %w{Alice Bob John Sarah}
ledger.transaction 400, 'Sarah', %w{Alice Bob John Sarah}
ledger.transaction 100, 'John', %w{Alice Bob}
# 
# puts ledger.report

# ledger = Ledger.new %w{John Sarah Katie Sam Joseph Ben Charlie Fred}, [50, 125, 15, -7, -6, -9, -43, -125]

puts ledger.report